1.1 Introduction
Before addressing the reproduction of colour it is essential to have firmly based ideas about what colour is, its spectral characteristics, the way it is described differently by different people and the importance of a common naming nomenclature. Thus this chapter describes how colour is perceived, and how it is unambiguously characterised, both in terms of the quantitative and qualitative responses it evokes in the eye.
The sensitive elements of the eye at normal levels of illumination are identified and characterised both in terms of their overall sensitivity and in terms of their spectral responses.
1.2 Setting the Scene
To introduce colour as a subject for study immediately presents one with a problem. In contrast to other subjects we may decide to investigate, we all have preconceptions as to what colour is. We think we already know much about colour, we have experienced it from early childhood, colour names crop up in speech on a regular basis, we have probably been taught at school how to mix colours to obtain a wider range than those available in the paint box and almost certainly at some stage we have been introduced to the concept of primary colours as the basis of obtaining a wide range of colours from the mixture in varying amounts of just three distinctly different primary hues. We will be formally defining the parameters which are used to describe the various aspects of colour later. For the present, however, the hue of a colour describes whether it is, for example, green, yellow or violet.
However the manner in which we perceive colour, though at an overview level not particularly complex, is just complex enough to require a level of attention beyond that which many of us have been prepared to give on a casual basis. Experience has shown that as a result there is widespread confusion about how colour is perceived.
One of the problems associated with initial considerations of colour perception is the naming of colours and the manner in which we differentiate colours of various hues through the spectrum. The following four paragraphs on unitary hues adapted from the work of Ray Knight provide a sound basis on which to commence consideration of this topic.
As we step through the visible spectrum from red to violet we pass a considerable number of quite distinct hues. A listing might read as: red, orange, yellow, green, cyan or turquoise, blue and violet. (Purple and magenta hues do not appear in the spectrum.) Because these seven colours continuously blend into each other we perceive many more than these seven hues, and certainly more hues than we have distinct colour names to identify them with.
Of the seven fundamental colours named above, only four are truly distinct and to these we can add black and white which together make up a group called psychological colours or, when hues only are being referred to, as unitary hues, attributed to Ewald Hering (1834–1918), which are red, yellow, green and blue. These four important colours, share the distinction that each one can be described without reference to the other three, or any other colour. Consider yellow, for example. To find a pure yellow without a hint of either adjacent colour means we look for a yellow with an absence of green and an absence of red – perhaps chrome yellow. Such a hue can be found, but not so with orange or purple and some other hues. Orange has within it an element of yellowness and redness, and purple ha...
